This invention relates to a rotary valve and is primarily concerned with rotary valves of the type utilised for controlling fluid (usually hydraulic) to two opposed pressure chambers of a double acting power assistance device of a steering gear. Rotary valves for power assisted steering gears are well known in the automotive industry and conventionally comprise a rotor mounted in the bore of a sleeve so that during a steering manoeuvre the rotor and sleeve are rotationally displaced relative to each other and in so doing direct fluid through control ports in the valve to and from the power assistance means as appropriate to assist the steering manoeuvre. The power assistance means will usually be in the form of a double acting piston and cylinder device incorporated in the steering gear. Examples of rotary valves of the type aforementioned are to be found in U.K Pat. Nos. 391,775, 476,590, 1,356,172 and 2,028,240 and U.S. Pat. Nos. 1,947,973, 2,328,312, 1,657,412 and 1,773,794. In each of these examples fluid flow through the valve is achieved by providing distributing zones on the rotor which are displaced during the aforementioned relative rotation to determine fluid flow through the ports in the valve, such ports being connected to fluid pressure supply, the power assistance means and a low pressure return (exhaust/reservoir). Since the early 1930s the most favoured sleeve structure has comprised a peripherally spaced array of axially extending, blind ended, recesses in the bore of the sleeve. These recesses constitute ports in the valve and whilst this popular design is efficient and reliable in operation, it suffers from two serious disadvantages. Firstly, the design does not lend itself to economic manufacture on a mass production basis where either several assembly stages are required for producing the sleeve as a two or a three part component as envisaged by the disclosure in U.K. Pat. No. 476,590 and U.S. Pat. No. 1,947,973, or expensive special purpose machinery is necessary for manufacturing the sleeve as a one piece component as envisaged by the disclosure in U.S. Pat. No. 2,328,312 and U.K. Pat. No. 1,356,172. Secondly, the provision of the axially extending blind ended recesses calls for a relatively large sleeve and a correspondingly large housing within which the sleeve and rotor are mounted. This is incompatible with the desirability of providing a small, compact, steering gear as called for in modern vehicles. Although it has hitherto been proposed, for example in U.K. Pat. No. 2,028,240 to provide a sleeve for a rotary valve without blind ended axially extending recess type control ports, it has been found with such a proposal (as in many of the rotary valve proposals having the blind ended axially extending recesses) that a large number of ports and complex array of passages are necessary in the sleeve and/or the rotor to achieve the required fluid flow and also to ensure that the back-pressure in the valve is minimised when the valve is in its neutral condition. On this latter point, it will be seen by way of example that the rotary valve in U.K. Pat. No. 2,028,240 has twenty one ports of which fifteen have associated passages in the rotor and sleeve so in addition to providing a relatively complex hydraulic system, the manufacturing cost of such a valve is considerable.
It is an object of the present invention to provide a rotary valve for a power assisted steering gear which valve is relatively simple in construction, may employ relatively few ports and passages in the sleeve and/or rotor, can be of a design which minimises back pressure when in its neutral condition and will lend itself to economic manufacture as a compact assembly without loss of efficiency or reliability as compared with conventional forms of rotary valves for power assisted steering.